Most integrated circuits are fabricated using photolithographic methods. An exemplary procedure is illustrated by the following steps. A silicon substrate is covered by a layer of a light sensitive coating, usually positive photoresist. Photoresist is a light sensitive material that polymerizes or de-polymerizes, or in other words changes chemically, when it is exposed to light. Positive photoresist de-polymerized by light is soluble in a particular solvent and can be washed away. Positive photoresist masked from light remains polymerized and is thus insoluble in the solvent and cannot be washed away. Negative photoresist polymerized by light is insoluble in the solvent and cannot be washed away. Negative photoresist that is masked from light remains unpolymerized and can thus be washed away, leaving the polymerized photoresist in the shape of the negative of the original mask pattern.
Using positive photoresist as an example, an integrated circuit is fabricated by placing a transparent mask having an opaque mask pattern on its lower surface on top of the positive photoresist. After the silicon substrate has been covered by the photoresist and the mask, the photoresist and the mask are exposed to incident light which de-polymerizes the exposed photoresist. The de-polymerized positive photoresist is then washed away leaving the polymerized photoresist in the shape of the original mask pattern. Washing away the de-polymerized positive photoresist exposes the silicon substrate over areas outside the mask pattern. As a result, the silicon is exposed for such semiconductor processes as doping, material deposition, etching, ion implantation, oxidation, etc. The areas covered by the polymerized positive photoresist are shielded from these processes. The quality of the circuit resulting from this procedure depends upon the precision and resolution of the pattern of polymerized photoresist.
The quality of the pattern produced in the photoresist by the procedure described above degrades if directly reflected or diffuse reflected light polymerizes the photoresist in areas under the mask pattern. Improperly polymerized positive photoresist is washed away to expose areas of the substrate that should be covered. The problem is illustrated in FIGS. 1A and 1B. With reference to FIG. 1A, a layer of positive photoresist 10 coats the surface of a silicon substrate 12. A transparent mask 14 having an opaque mask pattern 16 formed on its lower surface is placed on the coating of photoresist 10. The mask pattern 16 is typically a very thin layer of chromium bonded to the surface of the transparent mask 14. With reference to FIG. 1B, the transparent mask 14 is exposed to incident light 18. Unwanted exposure of the coating of photoresist 10 can occur where rays of the incident light 18 pass through the coating of photoresist 10 (photoresist is generally transparent) and reflect from the surface of the silicon substrate 12. The incident light 18 reflecting from the surface of the silicon substrate 12 exposes the coating of photoresist 10 under the mask pattern 16, thereby causing the edges of the exposed portion of the coating of photoresist 10 to be indistinct or fuzzy. However, since the silicon substrate 12 is opaque, the incident light 18 reflecting from the upper surface of the silicon substrate 12 travels through only the coating of photoresist 10 before being reflected upward. Since the coating of photoresist 10 is relatively thin, the transverse distance traveled by the incident light 18 reflecting from the surface of the silicon substrate 12 is relatively short. Therefore, the reflected portion of the incident light 18 does not travel far enough in a transverse direction to affect more than a narrow fringe of the coating of photoresist 10 under the edges of the mask pattern 16. This small amount of excess polymerization does not substantially impair the resulting circuit.
Although the transverse shift of incident light does not present a significant problem for photolithography on an opaque silicon substrate, it does cause problems for photolithography on a transparent substrate. With reference to FIG. 2A, a coating of photoresist 20 covers the upper surface of a transparent substrate 22 that is supported by a backing plate 24. A transparent mask 26 having an opaque mask pattern 28 formed on its lower surface is placed over the coating of photoresist 20. With reference to FIG. 2B, the transparent mask 26 is exposed to incident light 30. The incident light 30 passes through the transparent mask 26, the coating of photoresist 20, and the transparent substrate 22, and is incident on the surface of the backing plate 24. Unfortunately, some of the incident light 30 reflects from the surface of the backing plate 24. Since the transparent substrate 22 may be substantially thicker than the coating of photoresist 20, the transverse distance traveled by the portion of the incident light 30 reflecting from the backing plate 24 may be substantial. As a result, when a photolithographic procedure is performed on a conventional transparent substrate, significant areas of the coating of photoresist 20 under the mask pattern 28 may be exposed. The reflecting portion of the incident light 30 exposing areas of the coating of photoresist 20 under the mask pattern 28 may be either non-diffuse light reflecting from the backing plate 24 having a smooth surface or diffuse light which reflects from the backing plate 24 having either an irregular surface or foreign particles or film on its surface.
In theory, purely columnar incident light does not present the problem described above because the columnar incident light reflects directly from the backing plate 24, and thus exits the coating of photoresist 20 over the same area that it entered. However, most incident light is not purely columnar and has transverse components. Also, as mentioned above, an irregular or dirty surface on the backing plate 24 may also diffuse purely columnar light back toward the masked portions of the coating of photoresist 20.
Several solutions to this problem mitigate but do not eliminate the reflection of light toward the masked portions of the coating of photoresist 20. Anodized aluminum or other darkened surfaces can be used on a light-absorbing backing plate 24 for transparent substrates. However, such surfaces do not absorb 100% of incident light. As a result, even dark surfaces reflect some incident light 30 back toward the masked portions of the coating of photoresist 20.
Attempts have also been made to solve the above-described problem by using columnar light and a backing plate 24 having a mirrored surface. However, as mentioned above, foreign matter such as particles or film inevitably contaminate the surface of the mirrored backing plate 24 and diffuse incident light toward the masked portions of the coating of photoresist 20. Any irregularities on the surface of the mirror would also reflect diffuse light toward the masked portions of the coating of photoresist 20. Finally, as mentioned above, the incident light 30 is generally not purely columnar and has transverse components as it reflects from the mirrored backing plate 24. As a result, such a mirrored surface on the backing plate 24 may exacerbate the problem of an improper polymerization of the masked portions of the coating of photoresist 20.
Although most photolithography is accomplished by placing a mask pattern over a coating of photoresist on a substrate and exposing the mask pattern to a source of incident light, a similar light pattern may also be generated directly by a projection printer. A projection printer projects a light pattern onto the coating of photoresist. This method results in the same kind of problems described above with respect to photolithography using a mask. With reference to FIG. 3, a transparent substrate 32 is positioned on a mirrored backing plate 34, and is covered with a coating of photoresist 36. Incident light 38 is projected from a projection printer 40 onto the coating of photoresist 36. The incident light 38 propagates through the coating of photoresist 36 and through the transparent substrate 32 to reflect off of the mirrored backing plate 34. Projection printers generally use light that is not perfectly columnar, as shown by the numeral 38 in FIG. 3, and therefore the reflections from the mirrored backing plate 34 travel a substantial transverse distance from the original incidence. Also, of course, even if the projected light was columnar, it would still be diffused by dirt, film, or other foreign matter on the surface of the mirrored backing plate 34. As a result, the reflected light from the projection printer produces fuzzy or indistinct edges in the resulting photoresist pattern.
The above-described procedures have been explained with reference to a positive photoresist in which the area of the photoresist exposed to the incident light becomes soluble in solvent as mentioned above. However, the discussion is equally applicable to the use of a negative photoresist in which the area of the photoresist exposed to the incident light becomes insoluble. As a result, the areas of negative photoresist corresponding to the mask pattern remain soluble and are washed away, leaving the silicon exposed in the shape of the mask pattern.